1. Field of Invention
The present invention relates to a control circuit for reducing output ripple in a constant on-time switching regulator and a method thereof, wherein when the circuit is in light load condition, the on-time is shortened for reducing the output ripple, while the whole circuit still operates in a discontinuous conduction mode (DCM).
2. Description of Related Art
Switching regulators can be classified into two types according to how its power switch is controlled: pulse-width modulation type and frequency modulation type. In a frequency modulation type switching regulator, a circuit controlling the power switch generates a constant ON time so that the power switch is turned ON for the same time period every cycle. The whole circuitry, by modulating the frequency, adjusts the level of an output voltage to regulate the voltage to a target value. Please refer to FIG. 1, wherein a buck switching regulator is taken as an example. A prior art constant on-time (COT) switching regulator includes: a zero current detector (ZCD) 70 detecting whether an inductor current iL is about zero and generating a zero current detecting signal Vzc; a comparator 80 comparing a feedback signal Vfb representing an output voltage Vout with a reference voltage Vref to generate a feedback control signal Vc; a one-shot pulse generator 60, wherein when the feedback signal Vfb is lower than the reference signal Vref, the one-shot pulse generator 60 is controlled by the feedback control signal Vc to generate a one-shot pulse signal with a COT; and a driver 10 controlling a power stage 20 according to the one-shot pulse signal to convert an input voltage Vin to the output voltage Vout, which is supplied to a load 100. The comparator 80 may be replaced by an error amplifier generating an analog output signal.
The prior art has a drawback that the pulse generated by the one-shot pulse generator 60 has a fixed on-time, that is, in each cycle a fixed amount of power is transferred from the input side to the output side. Thus, when the circuit operates in a very light load condition, too much energy is accumulated in the output capacitor C so that the circuit generates high output ripple.
In view of the above, the present invention proposes a control circuit for reducing output ripple in a COT switching regulator and a method for reducing output ripple in a COT switching regulator.